This invention relates to operational amplifiers, and more particularly to multi-operational amplifier systems.
Conventional operational amplifiers have limited common input ranges. Conventional complementary input operational amplifiers have a larger input common mode range, but the range is limited dependent on the threshold voltages of the input transistors of the operational amplifier. This range is described in conjunction with FIG. 1.
FIG. 1 is a graph illustrating the input-output transfer characteristics of conventional operational amplifiers. A P line 101 having a sloped portion and a horizontal portion illustrates the input-output transfer characteristics of a conventional operational amplifier having an input differential pair that includes a PMOS transistor pair. A Z-shaped N line 102 illustrates the input-output transfer characteristics of a conventional operational amplifier having an input differential pair that includes an NMOS transistor pair. As shown in FIG. 1, the PMOS based operational amplifier and the NMOS based operational amplifier each have a limited common input range.
Furthermore, in a mixed mode system, the usage of a particular operational amplifier configuration may be different for different analog paths, e.g., as a fixed reference unity buffer, a gain amplifier, and a rail-to-rail buffer. The usage also may be different at different times, e.g., as an input path or output path.